This project is focused on understanding the structural and functional changes that accompany the competition between axons to be the dominant source of innervation to their targets. The experiments focus on the well-characterized synapses of the neuromuscular junction that undergo a transition from polyinnervation to single innervation in early postnatal life. The basic approach is to view synaptic rearrangements at individual synaptic sites using transgenic mice with neuronal expression green fluorescent protein followed by complete serial electron microscopic reconstruction of these same synapses in the electron microscope. This approach will be used to not only make straightforward comparisons of optical and ultrastructural data, but to also assay for activity-dependent probes and migration of neurotransmitter receptors at the subcellular level. Using these imaging techniques, the specific aims of the proposal are to: 1) Determine whether all competing axon branches are functional during synapse elimination at the developing neuromuscular junction. By assaying for activity-dependent probes during serial electron microscopic reconstruction, this set of experiments will test the hypothesis that some axon branches are not functional during synaptic competition. 2) Establish how a dominant axonal input takes synaptic territory from a weaker competitor. Here, experiments are proposed to understand the cellular mechanisms that permit one axon to establish synapses in synaptic territory that is seemingly hostile to its competitor. 3) Follow migration of neurotransmitter receptors as receptor sites disappear during synapse elimination. This set of experiments is focused on understanding whether postsynaptic receptor loss is caused by internalization of postsynaptic sites or migration to extrasynaptic regions. Upon completion, the proposed experiments will make an original and important contribution to understanding the mechanisms whereby experience rewires the nervous system during development and possibly during learning and memory formation in adults.